1. Field of the Invention
This invention relates to synthesis of crystalline aluminosilicates of the molecular sieve type utilizing a treated smectite as a source of silica and alumina.
2. Description of the Prior Art
It has heretofore been known to employ certain clays, principally those of the kaolin type, in synthesizing crystalline aluminosilicate zeolites as a result of treatment with caustic solution. Thus, U.S. Pat. No. 2,992,068 describes conversion of dehydrated kaolin clay by contact with a relatively concentrated caustic solution at low temperature into a type A zeolite. U.S. Pat. No. 3,414,602 describes conversion of calcined kaolin by reaction with sodium hydroxide and tetramethylammonium hydroxide to yield zeolite N. U.S. Pat. Nos. 3,515,681 and 3,515,682 describe techniques for transforming calcined kaolin into synthetic faujasite by alkaline aging in sodium hydroxide solution. U.S. Pat. No. 3,037,843 describes treatment of a variety of clay minerals, particularly of the kaolin type, with caustic solution to yield a crystalline sodium aluminosilicate which is then treated with an acid to yield a "permutitic acid" which upon treatment with dilute alkali solution at an elevated temperature yields a crystalline aluminosilicate of the molecular sieve type. U.S. Pat. No. 3,508,867 describes treatment of clays with sodium hydroxide or potassium hydroxide solution, preferably in admixture with a source of silica, followed by calcination at temperatures ranging from 230.degree.F. to 1600.degree.F. to yield a material which is crushed, dispersed in water and digested in the resulting alkaline solution to yield, upon crystallization, a crystalline aluminosilicate. U.S. Pat No. 3,119,660 describes preparation of a molecular sieve-type crystalline aluminosilicate by contacting kaolin with an aqueous alkaline solution containing a water-soluble alkali metal salt. After a suitable period of digestion, crystallization occurs.
As noted, the clays previously employed for molecular sieve crystalline aluminosilicate preparation have almost invariably been of the kaolin type. Such clays have the general molar composition Al.sub.2 O.sub.3 . 2SiO.sub.2 . xH.sub.2 O. The SiO.sub.2 /Al.sub.2 O.sub.3 ratio may, however, vary from as low as 1.8 to as high as 2.6. The kaolin-type clays may be considered as sheet-like crystalline silicates. Their basic structural unit is an aluminosilicate sheet consisting of a layer of silicon cations in tetrahedral coordination with oxygen anions, bonded to a layer of aluminum cations in octahedral coordination with oxygen or hydroxyl anions. These sheets, of approximately 7 Angstroms thickness, are stacked one on top of another to form the small plate-like crystals of the mineral. Representative of kaolin-type clays which contain the above-described two-layer sheet structure are kaolinite, levisite, nacrite, dickite, endellite and halloysite. These clays differ only in the way the basic structural sheets are stacked.
Aside from the kaolin-type clays, there are large quantities of natural clay minerals of the smectite-type that have received essentally no attention as starting materials for zeolite synthesis. Those clays, also referred to as montmorillonite or bentonite-type, are composed of units made of two silica tetrahedral sheets with a central alumina-containing octahedral sheet. In the stacking of the silica-alumina containing silica units, oxygen layers of each unit are adjacent to oxygens of the neighboring units with a consequence that there is a very weak bond and an excellent cleavage between them. One of the outstanding features of such structure is that water and other polar molecules can enter between the unit layers causing the lattice to expand. These layers are 10 Angstroms thick compared to the 7 Angstroms layer thickness of the kaolin-type clays. The reason for the difference in plate thickness is the presence in the smectite type clays of the additional silicon tetrahedral sheet. The smectite type clays characteristically have a higher silica/alumina ratio than the kaolin-type clays.
The lack of attention accorded clays of the smectite type as sources of silica and alumina in crystalline aluminosilicate synthesis has been due, at least in part, to the high density and poor diffusion characteristics associated with these clays after calcination.